Light Modulator

ABSTRACT

A light modulator having a reduced parasitic static capacitance includes a semiconductor substrate having a mesa section and a bonding pad forming section formed thereon. A primary insulating film is formed on the substrate so as to continuously cover the mesa section and the bonding pad forming section. After a mask has been formed on a portion of the primary insulating film that is above the bonding pad forming section, the remaining portion of the primary insulating film is etched off, followed by removal of the mask. After the removal of the mask, a secondary insulating film is formed so as to continuously cover that portion of the primary insulating film above the bonding pad forming section and the mesa section so that a relatively thick insulating layer can be formed only above the bonding pad forming section.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a light modulator formodulating a laser beam and, more particularly, to a high speed lightmodulator of a kind used in a high speed optical fiber communicationsystem. The present invention also relates to a method of manufacturingsuch light modulator.

[0003] 2. Description of the Prior Art

[0004] In the high speed optical fiber communication system, aconsiderable amount of data are transmitted by the use of semiconductorlaser beams and optical fibers. In order to cope with this feature, thesemiconductor laser beams are required to be modulated at a high speed.With the conventional direct modulation system in which the electriccurrent injected to a single-mode semiconductor laser is modulated toprovide the modulated output laser beam, change in wavelength resultingfrom change in density of injected carriers (i.e., wavelength chirping)is so substantial that the conventional direct modulation system cannotbe used in high-speed modulation of 10 Gbps or higher.

[0005] In view of the foregoing, as an alternative to the directmodulation system, the external modulation system has come to be thecynosure of those concerned, in which a light modulator having a lowchirping and disposed externally of a semiconductor laser is utilized tomodulate the laser beam while the current injected to the semiconductorlaser is fixed. The combined modulator and laser assembly in which alight modulator, a single-mode semiconductor laser and an isolatorseparating the light modulator and the semiconductor laser from eachother are integrated together on a single chip is shown by 60 in FIG. 7.Since no circuit is required between the modulator and the laser, thecombined modulator and laser assembly 60 shown therein has a highpractical utility and is extremely important as a key device for opticalfiber communication of a large amount of data.

[0006] The light modulator will now be described. As shown in FIG. 8A,the light modulator 70 includes an InP semiconductor substrate 52 onwhich a semiconductor mesa layer 56 of a predetermined width containinga light absorption layer 51 and a semiconductor bonding pad layer 55 areformed. The laser beam inputted to the light modulator 70 is modulatedby the light absorption layer 51. More specifically, by applying avoltage to the bonding pad electrode 55 a, an electric field is appliedfrom an electrode 54, covering the semiconductor mesa layer 56, to thelight absorbing layer 51, and by shifting the absorption wavelength ofthe light absorbing layer 51, the input laser beam is modulated.

[0007] As shown in FIG. 8B, a groove 57 is formed between thesemiconductor mesa layer 56 and the semiconductor bonding pad layer 55for separating the semiconductor layers 55 and 56 from othersemiconductor layers. The semiconductor mesa layer 56, the semiconductorbonding pad layer 55 and the groove 57 has their respective surfacescovered by a continuous insulating film 53. The bonding pad electrode 55a and the electrode 54 are formed by a metallic film continuouslycovering the insulating film 53 while the electrode 54 is held in ohmiccontact with the semiconductor mesa layer 56 through an opening definedin the insulating film 53.

[0008] The conventional method of manufacturing the conventional lightmodulator is shown in FIGS. 9A to 9C. Referring first to FIG. 9A, apredetermined crystalline layer is epitaxially grown on the InPsubstrate 52 to form the semiconductor mesa layer 56 of thepredetermined width including the light absorption layer 51, the groove57 and the semiconductor bonding pad layer 55. Then, as shown in FIG.9B, the insulating film 53 of SiO₂ having a film thickness of about 4000Åis formed so as to cover the entire surface of the InP substrate 52.After a window for the ohmic contact has been formed in an upper surfaceof the semiconductor mesa layer 54 including the light absorption layer51, the metallic film is formed at a predetermined location as shown inFIG. 9C to complete the bonding pad electrode 55 a and the electrode 54.

[0009] In order for the light modulator to be used for high-speedmodulation, it is necessary to reduce the static capacitance(hereinafter referred to as a “parasitic static capacitance”) formedbetween surface electrodes (the bonding pad electrode 55 a and theelectrode 54) and a rear surface electrode. The parasitic staticcapacitance of the light modulator is expressed by the sum of theparasitic static capacitance of the mesa layer 56 plus the parasiticstatic capacitance of the bonding pad layer 55. In order to reduce theparasitic static capacitance of the light modulator, attempts havecurrently been made to minimize the surface area of each of the mesalayer 54 and the bonding pad layer 55 by forming the groove 57therebetween.

[0010] It has, however, been found that considering the chirping oflight that is propagated by the light absorption layer 51, the width ofthe mesa layer 56 can only be reduced to a certain limited dimension.Also, considering the bonding surface area of the bonding wire, the sizeof the bonding pad layer 55 is limited to about 50×50 μm. Thus, theapproach to reduce the surface area of the mesa layer 56 and the bondingpad layer 55 in an attempt to reduce the parasitic static capacitance islimited and, therefore, a sufficiently high-speed modulationcharacteristic has been difficult to accomplish.

SUMMARY OF THE INVENTION

[0011] The present invention has therefore been developed in view of theforegoing problems and is intended to provide an improved lightmodulator capable of accomplishing a high-speed light modulation inwhich the parasitic static capacitance is reduced and also to provide animproved method of manufacturing such light modulator.

[0012] The light modulator of the present invention is such that theparasitic static capacitance of the bonding pad section has been reducedto substantially eliminate the above discussed problems, and istherefore effective to achieve the high-speed modulation. Morespecifically, the light modulator of the present invention includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof. A mesa section of a predetermined width laminated with asemiconductor layer including a light absorption layer and a bonding padforming section adjacent the mesa section are formed on thesemiconductor substrate. An insulating layer continuing from the mesasection to the bonding pad section is formed with an opening defined ina portion of the insulating film above the mesa section, and anelectrode contacting an upper surface of the mesa section through theopening and extending to the bonding pad forming section is formed overthe insulating layer. Accordance with the present invention, the lightmodulator is featured in that a portion of the insulating layer thebonding pad forming section has a thickness greater than that of theremaining portion of the insulating layer to reduce the parasitic staticcapacitance of the bonding pad section.

[0013] The portion of the insulating layer immediately above the bondingpad forming section comprises a multi layered structure containing atleast insulating films laminated one above other. The remaining portionof the insulating layer comprises a single or multi layered structurecontaining a insulating films, in which a number of the insulating filmis less than that of the bonding pad forming section.

[0014] The insulating films are two insulating films, one of the twoinsulating films is made of SiO₂ and the other is made of SiN.

[0015] The upper-layer insulating film of remaining portion of theinsulating layer is same as the 2nd upper-layer insulating film of thebonding pad forming section.

[0016] The first method of manufacturing the light modulator accordingto the present invention is such that the parasitic static capacitanceof the bonding pad section has been reduced to substantially eliminatethe above discussed problems. More specifically, this first method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating film and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This first method is featured in thatit comprises forming a primary insulating film continuing from the mesasection to the bonding pad forming section, forming a mask so as tocover a portion of the primary insulating film formed above the bondingpad forming section, etching the primary insulating film to removeanother portion of the primary insulating film other than that portionof the primary insulating film above the bonding pad forming section,removing the mask forming a secondary insulating film continuing fromthat portion of the primary insulating film above the bonding padforming section and the mesa section and completing the insulatingwhereby that portion of the insulating layer above the bonding padforming section has a thickness greater than that of the remainingportion of the insulating layer to reduce the parasitic staticcapacitance of the bonding pad section.

[0017] The second method of manufacturing the light modulator accordingto the present invention is such that the parasitic static capacitanceof the bonding pad section has been reduced to substantially eliminatethe above discussed problems. More specifically, this second method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating film and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This second method is featured in thatit comprises forming a primary insulating film continuing from the mesasection to the bonding pad forming section, forming a mask so as tocover a portion of the primary insulating film other than a portion ofthe primary insulating film that is formed above the bonding pad formingsection, forming a secondary insulating film over the mask and thatportion of the primary insulating film above the bonding pad formingsection, removing the mask to allow that portion of the secondaryinsulating film above the bonding pad section to continue to thatportion of the primary insulating film above the mesa section to therebycomplete the insulating layer so that that portion of the insulatinglayer above the bonding pad forming section has a thickness greater thanthat of the remaining portion of the insulating layer to reduce theparasitic static capacitance of the bonding pad section.

[0018] The third method of manufacturing the light modulator accordingto the present invention is such that the parasitic static capacitanceof the bonding pad section has been reduced to substantially eliminatethe above discussed problems. More specifically, this third method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating layer and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This third method is featured in thatit comprises a primary insulating film forming step of forming a primaryinsulating film continuing from the mesa section to the bonding padforming section, forming a mask so as to cover a portion of the primaryinsulating film above the bonding pad forming section, etching anotherportion of the primary insulating film other than that portion of theprimary insulating film above the bonding pad forming section to apredetermined thickness, removing the mask so as to leave the insulatingfilm having a thick film portion above the bonding pad forming sectionand a thin film portion above the mesa section, the thick and thin filmportion being continued together, and completing the insulating layerwhereby that portion of the insulating layer above the bonding padforming section has a thickness greater than the remaining portion ofthe insulating layer to thereby reduce the parasitic static capacitanceof the bonding pad section.

[0019] The primary insulating film forming step of the third method ofthe present invention discussed above may include forming an under-layerinsulating film continuing from the mesa section to the bonding padforming section, forming over the under-layer insulating film anintermediate-layer insulating film of a material different from that ofthe under-layer insulating film, and forming over the intermediate-layerinsulating film an upper-layer insulating film of the same material asthat of the under-layer insulating film. In such case, the etching stepmay be carried out for selectively etching only a portion theupper-layer other than that formed above the bonding pad section, andcompleting the insulating layer.

[0020] Preferably, in any of the first to third method, the insulatingfilm is made of SiO₂ or SiN.

[0021] In the practice of the third method, one of the primaryinsulating film and the secondary insulating film is preferably made ofSiO₂ while the other thereof is preferably made of SiN.

[0022] Also, in the practice of any one of the first to third methods ofthe present invention, the insulating film is preferably formed by theuse of a CVD technique, a sputtering technique or a vacuum evaporationtechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1A is a fragmentary perspective view of a light modulatoraccording to a first preferred embodiment of the present invention;

[0024]FIG. 1B is a cross-sectional view taken along the line IB-IB inFIG. 1A;

[0025]FIGS. 2A to 2D are respective views similar to FIG. 1B, showing amethod of manufacture of the light modulator according to the firstembodiment of the present invention;

[0026]FIGS. 3A to 3D are respective views similar to FIG. 1B, showingthe modified method of manufacture of the light modulator according tothe first embodiment of the present invention;

[0027]FIGS. 4A to 4D are respective views similar to FIG. 1B, showingthe further modified method of manufacture of the light modulatoraccording to the first embodiment of the present invention;

[0028]FIG. 5A is a fragmentary perspective view of a light modulatoraccording to a second preferred embodiment of the present invention;

[0029]FIG. 5B is a cross-sectional view taken along the line VB-VB inFIG. 5A;

[0030]FIGS. 6A to 6D are respective views similar to FIG. 5B, showingthe method of manufacture of the light modulator according to the secondembodiment of the present invention;

[0031]FIG. 7 is a perspective view of the prior art combined modulatorand laser assembly;

[0032]FIG. 8A is a perspective view of the prior art modulator;

[0033]FIG. 8B is a cross-sectional view taken along the line VIIIB-VIIIBin FIG. 8A; and

[0034]FIGS. 9A to 9C are view similar to FIG. 8B, showing the prior artmethod of manufacturing the prior art light modulator.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

[0035] A light modulator according to a first embodiment of the presentinvention is shown by 50 in FIGS. 1A to 4D. Referring particularly toFIGS. 1A and 1B, the light modulator 50 includes a mesa section 9 and abonding pad forming section 6 both protruding outwardly from an InPsemiconductor substrate 2. More specifically, the mesa section 9 isdelimited by and between a pair of parallel grooves 3 formed at apredetermined position in the semiconductor substrate 2. The bonding padforming section 6 is positioned on one side of one of the grooves 3opposite to the mesa section 9 and is delimited by such one of thegrooves 3 and a generally U-shaped groove 3 a defined in thesemiconductor substrate 2. The mesa section 9 includes an electrode 5formed thereon through a SiO₂ insulating layer 4 and has a lightabsorption layer 1 embedded therein, which layer 1 is operable toreceive and transmit a laser beam therethrough. On the other hand, thebonding pad forming section 6 includes a bonding pad electrode 6 aformed thereon through a SiO₂ insulating thick-layer 40 interveningbetween it and the bonding pad electrode 6 a. The bonding pad electrode6 a and the electrode 5 form and are served respectively by differentparts of a metallic layer. It is to be noted that the light modulator 50has a rear surface formed with a rear surface electrode 10.

[0036] In the illustrated light modulator50, the insulating layer 4 andthe insulating thick-layer 40 are integral parts of a single insulatinglayer that are formed above the mesa section 9 and above the bonding padforming section 6, respectively. However, as best shown in FIG. 1B, aportion of the insulating layer overlaying the bonding pad formingsection 6, that is, the insulating thick-layer 40 has a thicknessgreater than the remaining portion of the insulating layer, for example,the insulating layer 4 overlaying the mesa section 9. In addition asdiscussed hereinabove, the insulating layer 4 and the insulatingthick-layer 40 are covered by the metallic layer and a portion of themetallic layer immediately above the bonding pad forming section 6serves as the bonding pad electrode 6 a while another portion of themetallic layer immediately above the mesa section 9 serves the electrode5. The electrode 5 is held in contact with an upper surface of the mesasection 9 through an opening defined in the insulating layer 4.

[0037] Since the insulating thick-layer 40 is formed only immediatelybelow the bonding pad electrode 6 a, the parasitic static capacitancebetween the bonding pad electrode 6 a and the rear surface electrode 10can be reduced. In detail, by increasing the thickness of thick-layer 40around the bonding pad forming section 6 from 4000 Åto 8000 Å,theparasitic static capacitance around the bonding pad forming section 6 ishalf of the parasitic static capacitance around the mesa section9,becase the capacitance is inverse proportion to the thickness of theinsulating layer. Also, since the insulating thick-layer 40 is formed ona relatively narrow portion of the bonding pad forming section 6, thereis no possibility that the substrate 2 may warp. Furthermore, since thatportion of the insulating layer overlaying the mesa section 9 has arelatively small thickness as is the case with that in the conventionallight modulator, there is no difficulty forming the opening in theinsulating layer.

[0038] The method of manufacturing the light modulator 50 of thestructure discussed above will now be described with particularreference to FIGS. 2A to 2D. At the outset, as shown in FIG. 2A, after apredetermined crystalline layer has been epitaxially grown on the InPsemiconductor substrate 2, a pair of grooves 3 are formed at apredetermined location in the semiconductor substrate 2 so that the mesasection 9 of a predetermined width having the light absorption layer 1can be eventually formed between these grooves 3. The generally U-shapedgroove 3 a continued to one of the grooves 3 is also formed to definethe bonding pad forming section 6. Then, as shown in FIG. 2B, a SiO₂insulating film 41 having a thickness of about 4000 Åis formed on thesemiconductor substrate 2 so as to overlay the mesa section 9 and thebonding pad forming section 6 continuously. Thereafter, as shown in FIG.2C, only a portion of the insulating film 41 overlaying the bonding padforming section 6 is covered by a photo-resist 7 that is utilized as amask, followed by removal of the remaining portion of the insulatingfilm 41 other than that portion of the insulating film 41 overlaying thebonding pad forming section 6. After the subsequent removal of thephoto-resist 7, as shown in FIG. 2D, another SiO₂ insulating film 42having a thickness of about 4000 Åis again formed on the semiconductorsubstrate 2 so as to overlay the mesa section 9 and the bonding padforming section 6 continuously. In this way, the bonding pad formingsection 6 is covered by the insulating thick-layer 40 of a thickness ofabout 8000 Å, i.e., that portion of the insulating film 41 overlapped bythe insulating film 42, while a portion other than the bonding padforming section, for example, the mesa section 9 is covered only by theinsulating film 42 of about 4000 Å.

[0039] Thus, the bonding pad electrode 6 a and the electrode 2 areformed immediately above the bonding pad forming section 6 and the mesasection 2, respectively. More specifically, while the opening is formedin the portion of the insulating layer overlaying an upper surface ofthe mesa section and the metallic layer connected in part with the uppersurface of the mesa section through the opening and in part overlayingthe bonding pad forming section. The rear surface electrode 10 is formedin any known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

[0040] Since as discussed above the insulating thick-layer 40 is formedonly immediately below the bonding pad electrode 6 a, the parasiticstatic capacitance between the bonding pad electrode 6 a and the rearsurface electrode 10 can be reduced. Also, since the insulatingthick-layer 40 is formed on that relatively narrow portion of thebonding pad forming section 6, there is no possibility that thesubstrate 2 may warp. Furthermore, since that portion of the insulatinglayer overlaying the mesa section 9 has a relatively small thickness asis the case with that in the conventional light modulator, there is nodifficulty forming the opening in the insulating layer.

[0041] The light modulator 50 of the present invention can also bemanufactured by an alternative method which will now be described withreference to FIGS. 3A to 3D. Referring to FIG. 3A, after a predeterminedcrystalline layer has been epitaxially grown on the InP semiconductorsubstrate 2, a pair of grooves 3 are formed at a predetermined locationin the semiconductor substrate 2 so that the mesa section 9 of apredetermined width having the light absorption layer 1 can beeventually formed between these grooves 3. The generally U-shaped groove3 a continued to one of the grooves 3 is also formed to define thebonding pad forming section 6. Then, as shown in FIG. 3B, a SiO₂insulating film 40 a having a thickness of about 8000 Åis formed on thesemiconductor substrate 2 so as to overlay the mesa section 9 and thebonding pad forming section 6 continuously. Thereafter, as shown in FIG.3C, only a portion of the insulating film 41 overlaying the bonding padforming section 6 is covered by a photo-resist 7 that is utilized as amask, followed by etching of the remaining portion of the insulatingfilm 40 a other than that portion of the insulating film 40 a overlayingthe bonding pad forming section 6 to render that remaining portion ofthe insulating film 40 a to have a film thickness of about 4000 Å. Afterthe subsequent removal of the photo-resist 7, as shown in FIG. 3D, theinsulating layer continuously covering the semiconductor surfaceincluding the bonding pad forming section 6 and the mesa section 9 canbe obtained. A portion of the insulating layer overlying the bonding padforming section 6 and its vicinity has a film thickness of about 8000Å(the insulating thick-film 40) while the other portion of theinsulating film, for example, a portion of the insulating filmoverlaying the mesa section has a thickness of about 4000 Å(theinsulating film 4).

[0042] Thus, the bonding pad electrode 6 a and the electrode 2 areformed immediately above the bonding pad forming section 6 and the mesasection 2, respectively. More specifically, while the opening is formedin the portion of the insulating layer overlaying an upper surface ofthe mesa section and the metallic layer connected in part with the uppersurface of the mesa section through the opening and in part overlayingthe bonding pad forming section. The rear surface electrode 10 is formedin any known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

[0043] Since as discussed above the insulating thick-layer 40 is formedonly immediately below the bonding pad electrode 6 a, the parasiticstatic capacitance between the bonding pad electrode 6 a and the rearsurface electrode 10 can be reduced. Also, since the insulatingthick-film 40 is formed on that relatively narrow portion of the bondingpad forming section 6, there is no possibility that the substrate 2 maywarp. Furthermore, since that portion of the insulating layer overlayingthe mesa section 9 has a relatively small thickness as is the case withthat in the conventional light modulator, there is no difficulty formingthe opening in the insulating layer.

[0044] A further modified method of manufacturing the light modulator 50of the present will now be described with reference to FIGS. 4A to 4D.At the outset, as shown in FIG. 4A, after a predetermined crystallinelayer has been epitaxially grown on the InP semiconductor substrate 2, apair of grooves 3 are formed at a predetermined location in thesemiconductor substrate 2 so that the mesa section 9 of a predeterminedwidth having the light absorption layer 1 can be eventually formedbetween these grooves 3. The generally U-shaped groove 3 a continued toone of the grooves 3 is also formed to define the bonding pad formingsection 6. Then, as shown in FIG. 4B, a SiO₂ insulating film 41 having athickness of about 4000 Åis formed on the semiconductor substrate 2 soas to overlay the mesa section 9 and the bonding pad forming section 6continuously. Thereafter, as shown in FIG. 4C, only a portion of theinsulating film 41 other than that overlaying the bonding pad formingsection 6, for example, only a portion of the insulating film 41overlaying the mesa section 9 is covered by a photo-resist 7 that isutilized as a mask, followed by deposition of a SiO₂ insulating film 42of about 4000 Åin thickness so as to continuously cover the mesa section9 and the bonding pad forming section 6. Accordingly, the insulatingfilm 42 is in part formed over the mask 7 and the bonding pad formingsection 6 is covered by not only the insulating film 41, but also theinsulating film 42 overlaying the insulating film 41. Subsequent removalof the photo-resist 7 is accompanied by removal of that portion of theinsulating film 42 overlaying the photo-resist 7, allowing that portionof the insulating film 41 overlaying the mesa section 9 to be exposed tothe outside. Consequently, as shown in FIG. 4D, the insulating layercontinuously overlaying the mesa section 9 and the bonding pad formingsection 6 can be obtained. Since that portion of the insulating film 41overlaying the bonding pad forming section 6 is laminated with acorresponding portion of the insulating film 42, that portion of theinsulating layer above the bonding pad forming section 6 has a thicknessof about 8000 Å(the insulating thick-layer 40), but the remainingportion of the insulating layer other than that over the bonding padforming section 6, for example, that overlaying the mesa section has afilm thickness of about 4000 Å(the insulating film 41).

[0045] Thus, the bonding pad electrode 6 a and the electrode 2 areformed immediately above the bonding pad forming section 6 and the mesasection 2, respectively. More specifically, while the opening is formedin the portion of the insulating layer overlaying an upper surface ofthe mesa section and the metallic layer connected in part with the uppersurface of the mesa section through the opening and in part overlayingthe bonding pad forming section. The rear surface electrode 10 is formedin any known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

[0046] Since as discussed above the insulating thick-layer 40 is formedonly immediately below the bonding pad electrode 6 a, the parasiticstatic capacitance between the bonding pad electrode 6 a and the rearsurface electrode 10 can be reduced. Also, since the insulatingthick-layer 40 is formed on that relatively narrow portion of thebonding pad forming section 6, there is no possibility that thesubstrate 2 may warp. Furthermore, since that portion of the insulatingfilm 42 overlaying the mesa section 9 has a relatively small thicknessas is the case with that in the conventional light modulator, there isno difficulty forming the opening in the insulating layer.

[0047] It is to be noted in any one of the foregoing methods, theinsulating layer 40 has been described as made of SiO₂, the presentinvention can be equally applied where SiN or any other insulating layeris employed therefor.

[0048] Also, formation of the insulating layer may be carried out anyknown method such as, for example, by the use of the CVD, sputtering orvacuum evaporation technique.

Second Embodiment

[0049] The light modulator 50 a according to a second preferredembodiment of the present invention is shown in FIGS. 5A and 5B. In thisembodiment, the light modulator 50 a includes, as shown in FIG. 5A, amesa section 9 and a bonding pad forming section 6 both protrudingoutwardly from an InP semiconductor substrate 2. More specifically, themesa section 9 is delimited by and between a pair of parallel grooves 3formed at a predetermined position in the semiconductor substrate 2. Thebonding pad forming section 6 is positioned on one side of one of thegrooves 3 opposite to the mesa section 9 and is delimited by such one ofthe grooves 3 and a generally U-shaped groove 3 a defined in thesemiconductor substrate 2. The mesa section 9 includes an electrode 5formed thereon through a SiO₂ insulating film 44 and has a lightabsorption layer 1 embedded therein, which layer 1 is operable toreceive and transmit a laser beam therethrough. On the other hand, thebonding pad forming section 6 includes a bonding pad electrode 6 aformed thereon through a SiO₂ insulating film 45 intervening between itand the bonding pad electrode 6 a. The bonding pad electrode 6 a and theelectrode 5 form and are served respectively by different parts of ametallic layer. It is to be noted that the light modulator 50 has a rearsurface formed with a rear surface electrode 10.

[0050] In the illustrated light modulator 50 a, as shown in FIG. 5B, adouble layered structure including a SiO₂ insulating film 43 and a SiNinsulating film 44 is formed on the semiconductor substrate 2 so as tocontinuously cover the mesa section 9, the bonding pad forming section 6and the generally U-shaped groove 3 a. The SiO₂ insulating film 45referred to above is formed over a portion of the SiN insulating film 44overlaying the bonding pad forming section 6. In other words, thebonding pad forming section 6 is covered by an insulating thick-film ofa three layered structure including respective portions of theinsulating films 43, 44 and 45. A metallic layer is formed over theinsulating film 4 and the insulating thick-film so that a portion of themetallic layer overlaying the bonding pad forming section and anotherportion of the metallic layer overlaying the mesa section 9 form thebonding electrode 6 a and the electrode 5, respectively. The electrode 5is held in contact with the mesa section 4 through an opening formedabove an upper surface of the mesa section 4.

[0051] Since the insulating thick-film is formed only immediately belowthe bonding pad electrode 6 a, the parasitic static capacitance betweenthe bonding pad electrode 6 a and the rear surface electrode 10 can bereduced. In detail, by increasing the thickness of thick-layer 40 aroundthe bonding pad forming section 6 from 4000 Åto 8000 Å, the parasiticstatic capacitance around the bonding pad forming section 6 is half ofthe parasitic static capacitance around the mesa section 9.Becase thecapacitance is inverse proportion to the thickness of the insulatinglayer. Also, since the insulating thick-film 40 is formed on arelatively narrow portion of the bonding pad forming section 6, there isno possibility that the substrate 2 may warp. Furthermore, since thatportion of the insulating layer overlaying the mesa section 9 has arelatively small thickness as is the case with that in the conventionallight modulator, there is no difficulty forming the opening in theinsulating layer.

[0052] The light modulator 50 a according to the second embodiment ofthe present invention can be manufactured by the following method whichwill be described with particular reference to FIGS. 6A to 6D. At theoutset, as shown in FIG. 6A, after a predetermined crystalline layer hasbeen epitaxially grown on the InP semiconductor substrate 2, a pair ofgrooves 3 are formed at a predetermined location in the semiconductorsubstrate 2 so that the mesa section 9 of a predetermined width havingthe light absorption layer 1 can be eventually formed between thesegrooves 3. The generally U-shaped groove 3 a continued to one of thegrooves 3 is also formed to define the bonding pad forming section 6.Then, as shown in FIG. 6B, a SiO₂ insulating film 43 of 2000 Åinthickness, a SiN insulating film 44 of 2000 Åin thickness and a SiO₂insulating film 45 of 4000 Åin thickness are successively formed on thesemiconductor substrate 2 so as to cover the mesa section 9 and thebonding pad forming section 6 continuously. Thereafter, as shown in FIG.6C, only the bonding pad forming section 6 is covered by a photo-resist7 that is utilized as a mask, followed by removal by etching of aportion of the outermost insulating film 45 overlaying the substrate 2,for example, the mesa section 9 and the grooves 3, other than thebonding pad forming section.

[0053] Since the insulating film 45 so removed is made of SiO₂ and theinsulating film 44 beneath the insulating film 45 is made of SiN, theuse of an etching solution capable of selectively etching SiO₂ onlywhile the insulating film 44 in the form of the SiN film serves as anetching stopper layer is effective to controllably remove only theoutermost SiO₂ film. For example, the etching soluitin is a hydrofluoricacid, because the etching rate for SiO₂ is as 3-5 times as for SiN.After the subsequent removal of the photo-resist 7, as shown in FIG. 6D,the insulating thick-film, about 8000 Åin thickness, of the threelayered structure including the insulating films 43, 44 and 45 areformed only over the bonding pad forming section 6, while the otherarea, for example, the mesa section 9 and the grooves 3 are covered bythe insulating film, about 4000 Åin thickness, of the double layeredstructure including the insulating layers 43 and 44.

[0054] Thus, the bonding pad electrode 6 a and the electrode 2 areformed immediately above the bonding pad forming section 6 and the mesasection 2, respectively. More specifically, while the opening is formedin the portion of the insulating layer overlaying an upper surface ofthe mesa section and the metallic layer connected in part with the uppersurface of the mesa section through the opening and in part overlayingthe bonding pad forming section. The rear surface electrode 10 is formedin any known manner as shown in FIG. 5A, thereby completing the lightmodulator 50 of the present invention.

[0055] It is to be noted in any one of the foregoing methods, theinsulating films 43 and 45 has been described as made of SiO₂ and theinsulating film 44 has been described as made of SiN, the presentinvention may not be always limited thereto and the insulating films 43and 45 may be made of SiN and the insulating film 44 maybe made of SiO₂.In this case, For example, the etching method is a plasma etching by CF₄gas, because the etching rate for SiN is over 5 times than for SiO₂.

[0056] In the practice of the method of manufacturing the lightmodulator 50 a, the insulating films or layers are preferably made bythe use of any known method such as, for example, by the use of the CVD,sputtering or vacuum evaporation technique.

[0057] Since as discussed above the insulating thick-film 45 is formedonly immediately below the bonding pad electrode 6 a, the parasiticstatic capacitance between the bonding pad electrode 6 a and the rearsurface electrode 10 can be reduced. Also, since the insulatingthick-layer 40 is formed on that relatively narrow portion of thebonding pad forming section 6, there is no possibility that thesubstrate 2 may warp. Furthermore, since that portion of the insulatinglayer overlaying the mesa section 9 has a relatively small thickness asis the case with that in the conventional light modulator, there is nodifficulty forming the opening in the insulating layer.

[0058] Thus, in the light modulator of the present invention, thatportion of the insulating layer immediately below the bonding padelectrode has a thickness greater than the remaining portion thereof toreduce the parasitic static capacitance of the bonding pad electrode.Accordingly, the light modulator so manufactured can be used for ahigh-speed modulation.

[0059] The parasitic static capacitance of the bonding pad electrode caneffectively be reduced as a result that that portion of the insulatinglayer immediately below the bonding pad electrode is made of SiO₂ orSiN. Accordingly, the light modulator so manufactured can be used for ahigh-speed modulation.

[0060] Since the light modulator of the present invention is such thatthe laminated structure of two or more insulating films is formedimmediately below the bonding pad electrode, the selective etchingmethod for forming the insulating layer immediately below the bondingpad electrode is effective to controllably form the insulating layer toa desired thickness.

[0061] According to the method of manufacturing the light modulator ofthe present invention, after the primary insulating film continuouslycovering the mesa section and the bonding pad forming section has beenformed, a portion of the primary insulating film other than thatcovering the bonding pad forming section is removed, followed byformation of a secondary insulating film continuously covering the mesasection and the bonding pad forming section. In this way, the insulatinglayer having a great thickness can be formed immediately below thebonding pad electrode.

[0062] Also, according to the method of manufacturing the lightmodulator of the present invention, after the primary insulating filmcontinuously covering the mesa section and the bonding pad formingsection has been formed, a mask is formed on a portion other than thebonding pad forming section, followed by formation of a secondaryinsulating film before the removal of the mask. In this way, theinsulating layer is formed in two stages only over the bonding padsection. In this way, the insulating layer having a great thickness canbe formed immediately below the bonding pad electrode.

[0063] Furthermore, according to the method of manufacturing the lightmodulator of the present invention, after the primary insulating filmcontinuously covering the mesa section and the bonding pad formingsection has been formed, a mask is formed on the bonding pad formingsection, followed by etching of a portion of the insulating film otherthan the bonding pad forming section to a predetermined film thickness.In this way, the insulating layer is formed in two stages only over thebonding pad section. In this way, the insulating layer having a greatthickness can be formed immediately below the bonding pad electrode atone step, not two step. Yet, according to the method of manufacturingthe light modulator of the present invention, the selective etchingtechnique is used to etch the primary insulating film covering the mesasection and the bonding pad forming section continuously. By thisreason, the film thickness of the insulating film can be accuratelycontrolled.

[0064] In the practice of the method of manufacturing the lightmodulator of the present invention, the insulating film or films aremade of SiO₂ or SiN and, accordingly the parasitic static capacitance ofthe light modulator can effectively be reduced.

[0065] Also, in the practice of the method of manufacturing the lightmodulator of the present invention, the insulating film or films aremade of SiO₂ and SiN and, accordingly the parasitic static capacitanceof the light modulator can effectively be reduced.

[0066] The use of the CVD, sputtering or vacuum evaporation technique toform the insulating film or films is effective to facilitate formationof the insulating film or films.

[0067] Although the present invention has been described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

What is claimed is:
 1. A light modulator comprising; a semiconductorsubstrate having a rear surface formed with a grounding conductor; amesa section of a predetermined width formed on said semiconductorsubstrate, said mesa section including a laminated layer structurehaving a light absorbing layer; a bonding pad forming section formed onsaid semiconductor substrate at a location adjacent said mesa section;an insulating layer formed on said semiconductor substrate so as tocontinuously cover said mesa section and said bonding pad formingsection, a portion of said insulating layer immediately above said mesasection having an opening defined therein; and an electrode formed oversaid bonding pad forming section and electrically connected with saidmesa section through said opening, a portion of said insulating layerimmediately above said bonding pad forming section having a filmthickness greater than the remaining portion of said insulating layer toreduce the parasitic static capacitance of said remaining portion. 2.The light modulator according to claim 1 , wherein the portion of saidinsulating layer immediately above said bonding pad forming sectioncomprises a multi layered structure containing at least insulating filmslaminated one above other, the remaining portion of said insulatinglayer comprises a single or multi layered structure containing ainsulating films, in which a number of said insulating film is less thanthat of said bonding pad forming section.
 3. The light modulatoraccording to claim 2 , wherein, said insulating films are two insulatingfilms, one of said two insulating films is made of SiO₂ and the other ismade of SiN.
 4. The light modulator according to claim 2 , wherein theupper-layer insulating film of remaining portion of said insulatinglayer is same as the 2nd upper-layer insulating film of said bonding padforming section.
 5. A method of manufacturing a light modulatorcomprising the steps of; forming a mesa section of a predetermined widthincluding a laminated layer structure having a light absorbing layer ona semiconductor substrate having a rear surface formed with a groundingconductor; forming a bonding pad forming section on said semiconductorsubstrate at a location adjacent said mesa section; forming a primaryinsulating film so as to continuously cover said mesa section and saidbonding pad forming section; forming a secondary insulating film over aportion of said primary insulating film above said bonding pad formingsection so that said mesa section has a number of said insulating filmis less than that of said bonding pad forming section; forming anopening in a portion of said primary insulating film immediately abovesaid mesa section; and forming an electrode over said bonding padforming section and electrically connected with said mesa sectionthrough said opening.
 6. The method according to claim 5 , wherein saidstep of forming said secondary insulating film comprising; forming maskover said primary insulating film other than said primary insulatingfilm above said bonding pad forming section; forming said secondaryinsulating film over said mask and said primary insulating film abovesaid bonding pad forming section; and removing said mask so as to allowa portion of said secondary insulating film above said bonding padsection to continue to a portion of said primary insulating film abovesaid mesa section to thereby complete said insulating layer in which adouble layered structure of insulating films are formed over saidbonding pad forming section and a single layered structure of insulatingfilm is formed over said mesa section.
 7. The method according to claim5 , wherein said step of forming said secondary insulating filmcomprising; forming a mask above a portion of said primary insulatingfilm above said bonding pad forming section; removing by etching saidprimary insulating film other than a portion of said insulating filmabove said bonding pad forming section; and, forming a secondaryinsulating film over said a portion of said insulating film above saidbonding pad forming section and above said masa section, whereby adouble layered structure including said primary and secondary insulatingfilm is formed above said bonding pad forming section and a singlelayered structure of said primary insulating film is formed above saidmesa section.
 8. The method according to claim 5 , wherein saidinsulating film is made of a material selected from said groupconsisting of SiO₂ and SiN.
 9. The method according to claim 5 , whereinsaid insulating film is formed by the use of a CVD technique, asputtering technique or a vacuum evaporation technique.
 10. A method ofmanufacturing a light modulator comprising the steps of: forming a mesasection of a predetermined width including a laminated layer structurehaving a light absorbing layer on a semiconductor substrate having arear surface formed with a grounding conductor; forming a bonding padforming section on said semiconductor substrate at a location adjacentsaid mesa section; forming a primary insulating film so as tocontinuously cover said mesa section and said bonding pad formingsection; forming a mask so as to cover a portion of said primaryinsulating film that is formed above said bonding pad forming section,etching to a predetermined thickness another portion of said primaryinsulating film other than that portion thereof above said bonding padforming section; and, removing said mask thereby leave said insulatinglayer having a first portion overlaying said bonding pad forming sectionand a second portion overlaying said mesa section, said first portion ofsaid insulating layer having a thickness greater than said secondportion of said insulating layer; forming an opening said primaryinsulating film immediately above said mesa section; and forming anelectrode over said bonding pad forming section and electricallyconnected with said mesa section through said opening.
 11. The methodaccording to claim 10 , wherein said insulating layer is made of amaterial selected from said group consisting of SiO₂ and SiN.
 12. Themethod according to claim 10 , wherein said step of forming said primaryinsulating film comprising the steps of forming an under-layerinsulating film continuing from said mesa section to said bonding padforming section, forming over said under-layer insulating filmanintermediate-layer insulating film of a material different from thatof said under-layer insulating film, and forming over saidintermediate-layer insulating film an upper-layer insulating film ofsaid same material as that of said under-layer insulating film, and saidsaid step of etchig comprising the step of selectively etching only aportion said upper-layer other than that formed above said bonding padsection.
 13. The method according to claim 12 , wherein one of saidprimary and secondary insulating films is made of SiO₂ and said otherthereof is made of SiN.
 14. The method according to claim 10 , whereinsaid insulating film is formed by the use of a CVD technique, asputtering technique or a vacuum evaporation technique.